Many exercise machines claim to be King when it comes to calorie consumption rates.
The truth about calorie consumption, however, is that it relates to YOUR ability to produce the work, not the machine's. To burn 800 calories per hour, for example, on a treadmill, bike, elliptical, etc requires a very high (almost unsustainable) intensity of exercise.
By spreading work over a wide range of muscles, rowing consumes calories in more muscles, allowing a high calorie consumption within the users limits of fatigue. Other exercises, recruiting fewer muscles, require far higher intensities or exercise to achieve the same calorie consumptions as rowing.